Dual-graphite cells have been proposed as electrochemical energy storage systems using graphite as both, the anode and cathode, whereas the electrolyte cations intercalate into the negative electrode and the electrolyte anions intercalate into the positive electrode during charge. On discharge, cations and anions are released back into the electrolyte. In this contribution, we present highly promising results for "dual-ion cells" based on intercalation of bis(trifluoromethanesulfonyl)imide anions into a graphite cathode from an ionic liquid-based electrolyte, namely N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr 14 TFSI). As the compatibility of this ionic liquid with graphitic anodes is relatively poor, metallic lithium and lithium titanate (Li 4 Ti 5 O 12 ) are used as anode. As both cations and anions participate in the charge/discharge reaction and other anode materials than graphite are possible, we propose the name "dual-ion cells" for these systems. The cell performance was studied in terms of cut-off voltage, temperature, cycling stability, self-discharge and rate performance. Depending on the cut-off voltage and temperature, coulombic efficiencies of more than 99 % and specific discharge capacities exceeding 100 mAh g −1 (based on graphite cathode weight) were achieved. Furthermore, this system provides an excellent cycling stability and capacity retention above 99 % after 500 cycles, outperforming reported organic solvent-based dual-graphite or dual-ion cells. Graphite is a redox-amphoteric intercalation host and therefore cations and anions can be electrochemically intercalated at different potentials yielding so-called donor-type or acceptor-type graphite intercalation compounds (GICs).1,2 Currently, the predominantly used donor-type GIC is LiC x . The LiC x /C x redox couple is the major active compound for state-of-the-art negative electrodes in lithium ion batteries. [3][4][5][6][7] Compared to the limited number of cationic intercalation guests, there is a broad spectrum of different anions capable to form acceptor-type GICs. Examples are hexa-or tetrafluoride guest species, e. g. PF 6− , AsF In 1938, Rüdorff and Hofmann developed the first ion transfer or rocking chair cell based on the shuttling of HSO 4 − anions between two graphite electrodes ( Figure 1a).14 This cell can be considered as the ancestor of the well-known lithium-ion cell, where lithium cations are transferred between two insertion electrodes during the charge/discharge process (Figure 1b). 3 In the 1990s, a rechargeable electrochemical energy storage system, using graphite as positive and negative electrode material in combination with a non-aqueous electrolyte has been introduced by and Carlin et al. 27,28 Carlin et al. investigated the reductive and oxidative intercalation of different cations and anions from ionic liquid-based electrolytes (without any additional lithium salt), such as 1-ethyl-3-methylimidazolium hexafluorophosphate (EMI + PF 6 − ). This system, a so-called dual-graphite cell, was bas...

We present highly promising results for the use of graphite as both electrodes in a “dual-carbon” cell. An ionic liquid-based electrolyte mixture allows stable and highly reversible ion intercalation/de-intercalation into/from the electrodes.

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We present a detailed study on the exact location and dynamics of Li ions in the garnet-type material Li(5)La(3)Nb(2)O(12) employing advanced solid state NMR strategies. Applying temperature-dependent (7)Li-NMR, (6)Li-MAS-NMR, (6)Li-{(7)Li}-CPMAS-NMR, (6)Li-{(7)Li}-CPMAS-REDOR-NMR as well as 2D-(6)Li-{(7)Li}-CPMAS-Exchange-NMR spectroscopy, we were able to quantify the distribution of the Li cations among the various possible sites within the garnet-type structure and to identify intrinsic details of Li migration. The results indicate a sensitive dependence of the distribution of Li cations among the tetrahedral and octahedral sites on the temperature of the final annealing process. This distribution profoundly affects the mobility of the Li cations within the garnet-type framework structure. Extended Li mobility at ambient temperature is only possible if the majority of the Li cations is accommodated in the octahedral sites, as observed for the sample annealed at 900 degrees C. Octahedrally-coordinated Li cations could be identified as the mobile Li species, whereas the tetrahedral sites seem to act as a trap for the Li cations, rendering the tetrahedrally-coordinated Li cations immobile on the time scale of the NMR experiments.

Electrochemical energy storage systems using graphite as both the negative and the positive electrode have been proposed as “dual-graphite cells”. In this kind of electrochemical system, the electrolyte cations intercalate into the negative electrode and the electrolyte anions intercalate into the positive electrode, both based on graphite, during the charging process. On discharge, cations and anions are released back into the electrolyte. So far, the systems proposed in literature are primarily based on Li + and PF 6 - intercalation/de-intercalation into/from graphite from non-aqueous organic solvent based electrolytes. As the positive electrode potential during charging always exceeds 4.2 V vs. Li/Li + , the organic electrolyte starts to decompose at these highly oxidizing conditions resulting in insufficient discharge/charge efficiencies. The replacement of organic solvent by ionic liquids (ILs) leads an increased stability of the electrolyte towards oxidation and thus to remarkably higher efficiencies as well as an increased cycling stability. In fact, ionic liquids provide extended anodic electrochemical stability and in addition, no solvent co-intercalation occurs in parallel to anion intercalation at high potentials. Here, we present highly promising results for “dual-ion cells” based on a graphite cathode and an ionic liquid based electrolyte, namely N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr14TFSI). As the compatibility of this IL with graphite anodes is poor, alternative anodes such as metallic lithium or lithium titanate (Li4Ti5O12, LTO) are used. Consequently, the “dual-graphite” cell is renamed to “dual-ion” cell. In addition, the calculation of the specific energy of these systems will be in the focus of the discussion.

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Electrochemical energy storage devices utilizing graphitic carbons as positive electrode material have been proposed as "dual-ion cells". In this type of electrochemical cell, the electrolyte does not only act as a charge carrier, but additionally as the active material. In the charge process, lithium ions are inserted/intercalated or deposited into/on the negative electrode, e. g. Li 4 Ti 5 O 12 , graphite or metallic lithium, and the electrolyte anions are intercalated into the graphite positive electrode. In the discharge process, both lithium ions and anions are released back into the electrolyte. We report herein on the intercalation of bis(trifluoromethanesulfonyl) imide anions (TFSI − ) into a graphite cathode from an ionic liquid-based, namely N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl) imide (Pyr 14 TFSI), electrolyte. We studied the influence of the graphite characteristics, such as the particle size distribution, specific surface area and ratio of basal plane to "non-basal plane" surface areas, on the electrochemical behavior of TFSI − anion intercalation. The rate performance, long-term cycling behavior and stability of dual-ion cells at high temperatures (60 • C) are also discussed. A specific discharge capacity exceeding 100 mAh g −1 can be achieved at discharge rates of up to 10C, when operating at 60 • C.Graphite is a redox-amphoteric intercalation host and therefore different types of cations and anions can be electrochemically intercalated at specific potentials yielding so-called donor-type or acceptortype graphite intercalation compounds (GICs). 1,2 There exists a broad range of anions which are capable of forming acceptor-type GICs, for example trifluoroacetate, 3 bis(trifluoromethanesulfonyl) imide 4-6 or perfluorooctanesulfonate. 7,8 The concept of the reversible anion intercalation into a graphitebased cathode, with the goal of application in an electrochemical energy storage system, was introduced in the early work of Rüdorff and Hofmann in 1938. 9 They proposed a so-called dual-graphite cell that was based on the shuttling mechanism of HSO 4 − anions between two graphite electrodes with concentrated sulfuric acid as the electrolyte. 9 The reversible anion intercalation from non-aqueous electrolytes, especially ionic liquid electrolytes, was first reported by In particular, the intercalation of PF 6 − anions into a graphite positive electrode was studied in detail by Seel and Dahn, using organic solvent-based electrolytes, such as sulfones or mixtures of carbonates. 14,15 A major drawback of these electrolytes is their limited stability against oxidation at the high working potentials of the graphite positive electrode, which may exceed 5 V vs. Li/Li + . As a rule, organic solvent-based electrolytes decompose, resulting in continuous electrolyte degradation and inadequate coulombic efficiency. [16][17][18] Another issue, related to anion intercalation from organic solvent-based electrolytes is the fact that solvent co-intercalation reactions into the layered graphite stru...

Abstract. Graphite is a redox-amphoteric intercalation host and thus capable to incorporate various types of cations and anions between its planar graphene sheets to form so-called donor-type or acceptor-type graphite intercalation compounds (GICs) by electrochemical intercalation at specific potentials. While the LiC x /C x donor-type redox couple is the major active compound for state-of-the-art negative electrodes in lithium-ion batteries, acceptor-type GICs were proposed for positive electrodes in the "dual-ion" and "dual-graphite" cell, another type of electrochemical energy storage system. In this contribution, we analyze the electrochemical intercalation of different anions, such as bis(tri-